EARLY LICHEN COLONIZATION ON EARTHQUAKE-INDUCED ROCKFALLS: IMPLICATIONS FOR LICHENOMETRY AND PALEOSEISMOLOGY

Lichenometry has widely been employed as a geochronological dating technique for estimating the exposure age of rock surfaces and landforms. It has also been used extensively in paleoseismic studies to estimate the timing and recurrence interval for prehistoric and historical shaking events. However, despite its widespread use, lichenometric dating has faced criticism stemming from a lack of understanding surrounding early phase lichen growth characteristics and uncertainties associated with its methodologies. Here, we characterize early phase lichen colonization on Canterbury Earthquake Sequence (CES) rockfall surfaces at Rāpaki Rock (Te Ahi-a-Tamatea) and Castle Rock (Te Tihi-o-Kahukura) in the Port Hills of Christchurch, New Zealand. To ensure a high degree of surface variability, 22 CES rockfall surfaces with a range of lithologies, aspects, and roughness characteristics were chosen. For each CES rockfall surface, the diameter (longest axis) for the five largest lichens of each identified morphology were measured. We identified nine primary lichen morphologies (4 crustose, 4 foliose, 1 fruticose) and calculated growth rates and diameter density plots for each. Our results highlight the succession and spatial distribution of early colonizing lichen species on rockfall surfaces of variable composition and roughness characteristics and report inter- and intra-species variability in growth rates. Crustose lichens displayed the slowest growth rates (0.238-0.703 mm/year) with unimodal diameter density peaks, while the foliose lichen population was much more robust with higher growth rates (1.034-2.300 mm/year) but high variability in diameter density. Rhizocarpon geographicum, a species commonly used in lichenometry, was influenced by the combined factors of aspect and lithology. Our results also suggest that the lag time for complete colonization for many of the observed species is greater than 9 years (the current duration of time since the CES), indicating that the resolution of lichenometry is less than previously reported. In addition, Structure from Motion (SfM) Photogrammetry was performed on representative rockfall surfaces to create a three-dimensional photomosaic to document the size and morphology of lichens and establish a baseline dataset to directly monitor early phase lichen growth rates. Indirectly calculated growth rates coupled with the SfM dataset of direct growth rates will enhance the understanding of the accuracy and utility of lichenometry as a dating technique and paleoseismic tool.